This application is based upon and claims priority of Japanese Patent Application No. 2001-011885, filed in Jan. 19, 2001, the contents being incorporated herein by reference.
1. Field of the Invention
The present invention relates to a compound semiconductor device manufacturing method.
2. Description of the Prior Art
In the semiconductor laser, since the active layer has a narrow energy band gap and its end face is exposed to the high optical density state in the laser oscillation, the optical absorption due to the nonluminous recombination is ready to occur at the end face. Then, when the operating output of the semiconductor laser is increased, an amount of optical absorption at the end face of the active layer is increased to then raise the temperature, and then the energy band gap is further reduced because of such temperature rise and thus an amount of optical absorption is increased further. In the end, the COD (Catastrophic Optical Damage) destruction is caused.
In order to suppress such COD destruction, it is known that the energy band gap of the active layer is widened by diffusing zinc (Zn) into the end face of the active layer of the semiconductor laser from the top.
The steps of forming the end face window structure of the semiconductor laser by diffusing the zinc will be explained with reference to FIGS. 1A to 1F hereunder. A cross section of the semiconductor laser along the resonator length direction is shown in FIGS. 1A to 1F.
First, as shown in FIG. 1A, the n-type cladding layer 102 formed of n-AlGaInP, the MQW active layer 103 formed of GaInP/AlGaInP, the p-type cladding layer 104 formed of p-AlGaInP, and the contact layer 105 formed of p-GaAs are formed in sequence on the n-GaAs substrate 101 by the MOVPE method.
Then, the diffusion preventing mask 106 of SiO2 is formed on the contact layer 105 by the CVD method, and diffusion windows 106a are formed near the end faces by patterning the diffusion preventing mask 106.
In turn, as shown in FIG. 1B, the contact layer 105 is etched via the diffusion windows 106a. 
Then, as shown in FIG. 1C, the ZnO/SiO2 film 107 in which zinc oxide (ZnO) and silicon dioxide (SiO2) are mixed by 50 wt % respectively and the cover film 108 formed of SiO2 are formed in sequence in the diffusion windows 106a and on the diffusion preventing mask 106 by the sputter method. Here, it is considered that SiO2 is needed by about 50 wt % to form the group III vacancies in the group III-V semiconductor layer.
Then, as shown in FIG. 1D, if Zn in the ZnO/SiO2 film 107 is diffused into the MQW active layer 103 via the diffusion windows 106a by the annealing process, window structures 109 as the Zn-diffused regions are formed on the laser end faces.
Then, as shown in FIG. 1E, the diffusion preventing mask 106, the ZnO/SiO2 film 107, and the cover film 108 are removed by etching, then the SiO2 passivation film (not shown) is formed by the CVD, and then the long stripe-like opening (not shown) is formed in the resonator length direction by patterning the SiO2 passivation film.
After this, as shown in FIG. 1F the p-side electrode 110 is connected to the contact layer 105 via the stripe-like opening, and the n-side electrode 111 is formed on the lower surface of the n-GaAs substrate 101. The laser beam is emitted in the direction indicated by an arrow in FIG. 1F.
Meanwhile, the increase of the energy band gap in the MQW layer due to the Zn diffusion depends on the Zn concentration as shown in FIG. 2. FIG. 2 shows the result measured by the photo luminescence (PL) method at the room temperature (25xc2x0 C.), and the increase of the PL wavelength shift shows the increase of the energy band gap in the MQW layer.
When the window structures as the Zn-diffused regions are formed in the S3 (Self-aligned Stepped Substrate)-type semiconductor laser in compliance with the steps shown in FIGS. 1A to 1F, the improvement of the COD level is not so found.
Therefore, when the Zn diffusion depth in the laser end face regions is evaluated by the SEM microphotograph, the zinc is diffused only up to about 0.15 xcexcm under the active layer 103. It is considered that the shallow diffusion causes the event that the COD level is not improved.
However, based on the experiment made by inventors of the present invention, it becomes apparent that, in the case that the ZnO/SiO2 film 107 in which ZnO and SiO2 are contained by 50 wt % respectively is used as the Zn diffusion source, the Zn diffusion position cannot be extended deeper even when the annealing diffusion time is prolonged.
In order to extend the lower end of the Zn-diffused region (referred to as the xe2x80x9cZn diffusion frontxe2x80x9d hereinafter) deeper, the inventors of the present invention have considered that the deeper recesses should be provided physically in the wafer. However, since the fabrication processes become complicated, this approach is not preferable.
It is an object of the present invention to provide a compound semiconductor device manufacturing method that is capable of deepening a Zn-diffusion position from a ZnO/SiO2 film to such extent that COD tolerate quantity of laser end face window structures can be increased rather than the prior art, or easily controlling a Zn diffusion front position by an annealing time of the ZnO/SiO2 film.
According to the present invention, the zinc oxide/silicon oxide mixed film (ZnO/SiO2 film) which contains zinc oxide by 70 wt % or more is deposited on the compound semiconductor layers, for example, the multi-layered structure semiconductor layers constituting the semiconductor laser, and then zinc is diffused into the compound semiconductor layers from the ZnO/SiO2 film by annealing.
It is considered that, in view of the formation of the group III element vacancies by the silicon oxide, a contained amount of zinc oxide in the ZnO/SiO2 film should be set to about 50 wt %. According to the experiment, if a contained amount of zinc oxide is set to almost 50 wt %, the Zn diffusion front becomes shallow and thus the COD level of the active layer of the semiconductor laser, for example, cannot be increased.
In contrast, it is confirmed by the experiment made by the inventors of the present invention that, if a contained amount of zinc oxide in the ZnO/SiO2 film is set to about 70 wt % or more, the Zn diffusion front can be extended deeper, control of the depth can be facilitated by controlling the temperature and the time, and a contained amount of silicon oxide can be set lower than 50 wt %.
As a result, the laser window structure can be formed by diffusing the zinc up to the deep position under the end face regions of the active layer of the semiconductor laser, and also the COD level can be increased.